This application is based on Patent Application No. 11-367001 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method of forming an optical glass element, particularly the present invention relates to a method of forming an optical glass element by pressing the glass pre-form.
2. Description of the Related Art
Conventional methods of forming a precision optical element, e.g., a glass lens, by holding a pre-form between a top and bottom mold via a bottom shell having a stage, applying heat, and applying pressure to the top and bottom molds holding the pre-form when a specific temperature has been attained, as disclosed in Japanese Laid-Open Patent Application No. 2-133327. According to this method, however, when the pre-form is under pressure by the mold, the pressing process only applies pressure to the pre-form which is held by the stage part on the exterior surface of the pre-form, and local stress is concentrated on the exterior surface of the pre-form due to the presence of the stage part, such that the glass flow in the diameter direction is hampered on the bottom mold surface side of the pre-form, causing dead space near the exterior surface of the bottom mold surface, and disadvantageously and uncontrollably distorting the surface of the exterior surface of the obtained glass element. The generation of surface distortion is particularly pronounced when the bottom mold has a convex shape. In such optical elements, the area in which the shape of the mold surface shape is not transferred in spite of the intention to transfer such shape, due to the generated surface distortion and the like, is referred to as deformation hereinafter in the present specification.
In order to eliminate these disadvantages, Japanese Laid-Open Patent Application No. 9-286622 discloses art for applying pressure when the pre-form is separated from the stage during the pressing process. This art is briefly described using FIG. 3. The device shown in FIG. 3(a) has a top unit 100 and bottom unit 200, wherein the top unit comprises a top mold 1 having a top shell 11 covering its exterior surface, and a top backplate 23 fixedly attached to these elements, and the bottom unit comprises a bottom mold 2, a holder 4 having a stage D for holding a pre-form 3 and covering the bottom mold, bottom shell 12 for covering the exterior surface of the holder, and a bottom backplate 22 fixedly attached to these elements. That is, in the art of the aforesaid disclosure, a pre-form 3 is held between a top mold 1 and a bottom mold 2 by a holder having a stage, heated (refer to FIG. 3(a)), and when a specific temperature has been attained, the bottom mold 2 is moved while rubbing the holder 4 (bottom shell) via a push rod A, such that the pre-form 3 is under pressure while separated from the stage D (refer to FIG. 3(b)).
When using this art, however, the pre-form is shifted when pressure is applied resulting from the pre-form separating from the stage directly before pressure is applied, such that the obtained optical element has eccentric thickness or eccentricity which presents a problem in regard to precise shape.
An object of the present invention is to provide a method for forming an improved optical glass element and an optical glass forming apparatus using this method.
An object of the present invention is to particularly provide a method for forming an optical glass element without deformation and having excellent precision of shape, and an optical glass forming apparatus using this method.
These objects are attained by a method for forming an optical glass element by pressure molding a glass pre-form using top and bottom molds, said method comprising a heating process for heating a glass pre-form held at a position separated from the top and bottom molds by a holding member within the top and bottom molds, a first pressing process for applying pressure while the glass pre-form is held by the holding member, and a second pressing process for moving the bottom mold independently in an upward direction and applying pressure.